The present invention relates to electron generators including a field emission cathode with electron-emitting micropoints associated with a grid biased by positive grid biasing means and an anode attracting electrons emitted by the cathode.
Field emission cathodes with electron-emitting micropoints were developed as electron generators a few years ago. The electrically conductive micropoints are formed on an appropriate conductive substrate and in cavities of an insulative layer covering the substrate, with their ends flush with a positively biased grid incorporating openings facing each cavity. The sharp tips of the micropoints produce local amplification of the electric field which encourages emission of electrons at room temperature from a threshold voltage in the order of 50 to 100 volts, depending on the construction of the array of micropoints.
Field emission cathodes of the above kind have good energy efficiency, better than that of a conventional source of electrons in the form of a tungsten filament heated to temperature from 1000 to 2000xc2x0 C. and emitting electrons by a thermoelectric effect. The power required to emit electrons is of the order of 10 W for a heated tungsten electrode. The power required to emit electrons is of the order of 0.2 W for a field emission cathode.
Another advantage of field emission cathodes is their reaction speed at the start and at the end of emission of electrons.
In prior art devices, during rest periods (periods of non-emission of electrons), the grid is biased by a null voltage, i.e. the grid is at the same potential as the cathode. During emission of electrons, the grid is biased to a positive voltage higher than the emission threshold voltage of the cathode.
These field emission cathodes with electron-emitting micropoints are satisfactory for generating electrons continuously or modulated at a low frequency.
For some applications, for example time-of-flight mass spectrometers, it is necessary to generate an intermittent flow of electrons in the form of packets of electrons with the shortest possible duration, in the order of one nanosecond, and the packets of electrons must contain the highest possible number of electrons. The intermittent flow of electrons is equivalent to a current of 1 mA to 2 mA.
In this case, field emission cathodes require generators capable of biasing the grid with electrical pulses with a duration of the order of one nanosecond and of sufficient amplitude to reach the emission threshold voltage of the cathode. It is difficult to produce electrical pulses of high amplitude and short duration and, because of the consumption of electrical energy in the capacitive component that the field emission cathode itself constitutes and whose plates are the grid and the substrate with the micropoints, the power required is not negligible. Note that the power consumed in this capacitive component increases with the square of the AC voltage applied to it and is proportional to its frequency.
The present invention addresses the problem of designing a new pulse mode electron generator with a field emission cathode that significantly reduces power consumption in pulse mode and significantly facilitates control of the cathode by an electrical pulse generator that is easier to implement.
Another object of the invention is to improve the pulse mode of the electron generator by reducing the duration of the rising and falling edges of the electron pulses.
To achieve the above and other objects, the invention provides a pulse mode electron generator including a field emission cathode with electron-emitting micropoints associated with a grid biased by means for positively biasing the grid and an anode attracting electrons emitted by the cathode, and wherein the means for positively biasing the grid generate a grid bias voltage taking:
a rest voltage value which, during non-emission of electrons, remains only slightly lower than the active bias voltage of the grid from which the anode receives a non-negligible flow of electrons from the cathode, or
an emission voltage value which, during emission of electrons, is higher than said active bias voltage of the grid.
Between the step of non-emission of electrons and the step of emission of electrons, the amplitude of the bias voltage is modified by an amount equal to the difference between the emission voltage and the rest voltage. That amplitude is therefore only a fraction of the emission voltage. The power consumed in each pulse cycle is therefore greatly reduced, because only this difference between the emission voltage and the rest voltage enters into the computation of the power, in which it is squared. It is also easier to produce a pulse with steep fronts if its amplitude is equal only to the difference between the emission voltage and the rest voltage. This simplifies implementation of the bias generator and this improves the waveform of the packets of electrons emitted by the generator.
In one embodiment, the rest voltage is slightly lower than the cathode emission threshold voltage. The resulting generator is of structure that is particularly simple, including no components other than those ordinarily used in field emission cathodes with electron-emitting micropoints.
Instead of this feature, or in addition to it, a transparent repulsive grid can be disposed between the cathode grid and the anode and biased slightly below the cathode emission threshold voltage, in order to repel towards the cathode electrons having insufficient energy during non-emission of electrons and to allow through electrons having sufficient energy during emission of
This prevents emission of electrons more reliably during non-emission steps and can make it possible to use a slightly higher rest voltage for permanent biasing of the grid.
In practice, the means for positively biasing the grid can generate a grid bias voltage including a permanent positive pre-bias voltage on which there is superimposed a positive bias voltage pulse component
To simplify implementation of the grid bias generator, the means for positively biasing the grid can include a permanent positive pre-bias voltage generator delivering said permanent positive pre-bias voltage in series with a positive pulse voltage generator delivering said positive bias voltage pulse component.
Said positive bias voltage pulse component preferably has an amplitude of only a few volts. It is easy to generate a pulse have a duration of one nanosecond and an amplitude of approximately 10 volts.
The flow of electrons emitted by generators of the above kind takes substantially the same form as the signal consisting of said positive bias voltage impulse component.